JP4192791B2 - Exhaust gas purification catalyst - Google Patents

Description

  The present invention relates to a catalyst for purifying exhaust gas discharged from an internal combustion engine, and more particularly to a NOx occlusion reduction type exhaust gas purifying catalyst.

  Conventionally, as a catalyst for exhaust gas purification of automobiles, a three-way catalyst for purifying by simultaneously oxidizing CO and HC in exhaust gas and reducing NOx has been used. As such a three-way catalyst, for example, a catalyst support layer made of γ-alumina is formed on a support base material such as cordierite, and platinum (Pt), palladium (Pd), rhodium (Rh), etc. are formed on this catalyst support layer. Those having a noble metal catalyst supported thereon are widely known.

On the other hand, in recent years, from the viewpoint of protecting the global environment, carbon dioxide (CO ₂ ) in exhaust gas discharged from internal combustion engines such as automobiles has become a problem. As a solution, so-called lean burn that burns fuel in an oxygen-excess atmosphere. Has been proposed. In this lean burn, since the fuel consumption is improved, the amount of fuel used is reduced, and as a result, the generation of CO ₂ as combustion exhaust gas can be suppressed.

  However, the conventional three-way catalyst is one that simultaneously oxidizes, reduces, and purifies CO, HC, NOx in the exhaust gas when the air-fuel ratio (A / F) is the stoichiometric air-fuel ratio (stoichiometric). In an oxygen-excess atmosphere, the oxidation reaction for purifying CO and HC is active, while the reduction reaction for purifying NOx is inactive, and NOx cannot be purified.

  Therefore, in lean burn, a system for NOx purification using exhaust gas as a reducing atmosphere has been developed by always burning under lean conditions with excess oxygen and temporarily changing to stoichiometric to rich conditions. In this system, an NOx occlusion reduction type exhaust gas purifying catalyst using a NOx occlusion material that occludes NOx in a lean atmosphere and releases NOx occluded in a stoichiometric to rich atmosphere has been proposed (for example, Patent Document 1). reference). If such a catalyst is used, the air-fuel ratio is controlled from the lean side so as to change from the lean side to the stoichiometric to rich side. Further, since it is purified by reacting with reducing components such as HC and CO, NOx can be efficiently purified even with exhaust gas from a lean burn engine.

JP-A-6-142458

However, the exhaust gas purifying catalyst used in the conventional method has a problem that the NOx occlusion activity decreases as it is used in high temperature exhaust gas at 600 ° C. or higher. This is because alkali metals, alkaline earth metals, etc. used as NOx occlusion materials react or dissolve in a catalyst carrier such as alumina or titania or cordierite as a substrate at a temperature of 600 ° C. or higher. This is considered to be for forming BaTiO ₃ , K ₂ Ti ₄ O ₉ , BaAlO _{x and the} like.

  An object of the present invention is to provide an exhaust gas purifying catalyst capable of maintaining a practically acceptable high purification rate even after durability.

In order to solve the above problems, according to the present invention, in the exhaust gas purification catalyst comprising a catalyst support layer, a noble metal catalyst supported on the catalyst support layer and a NOx storage material, the following formula is used as the NOx storage material:
K ₆ MgO ₄
The complex oxide represented by these is used.

  According to the exhaust gas purifying catalyst of the present invention, it is possible to suppress alkali metal or alkaline earth metal from reacting with or solid-dissolving with the catalyst support layer or the base material at the time of use at a high temperature, thereby suppressing a decrease in NOx occlusion performance. Can do.

  The exhaust gas purifying catalyst of the present invention comprises a catalyst supporting layer, a noble metal catalyst supported on the catalyst supporting layer, and a NOx storage material. The catalyst support layer is composed of an oxide porous body generally used as a carrier (or washcoat) in a conventional catalyst. Examples of such oxide porous body include alumina, silica, zirconia, silica-alumina, and zeolite. Etc. are used.

  Examples of the noble metal catalyst include platinum (Pt), rhodium (Rh), palladium (Pd), iridium (Ir), and ruthenium (Ru), which are conventionally used as a three-way catalyst. Multiple species can be used. The amount of the noble metal catalyst supported is preferably a normal amount supported, for example, 0.1 to 10 g / L with respect to the catalyst carrier. If the amount is less than 0.1 g / L, sufficient catalytic activity cannot be obtained, and if the amount exceeds 10 g / L, the activity is only slightly improved and only expensive. This noble metal catalyst can be supported on the catalyst support layer by a conventional method, for example, a precipitation method, an adsorption method, an ion exchange method, a reduction precipitation method, an evaporation to dryness method or the like.

The present invention uses the following formula as a NOx occlusion material
K ₆ MgO ₄
In the use of the composite oxide represented you characterized.

  This composite oxide is a general composite oxide manufacturing method, for example, a powder co-firing method in which each metal oxide or carbonate, a precursor powder such as hydroxide is mixed and fired, a plurality of metal inorganics Coprecipitation method to neutralize by adding alkali to aqueous salt solution to produce colloidal dispersion of oxide or hydroxide, Alkoxide method to hydrolyze by adding water to multiple metal alkoxides dissolved in organic solvent , Etc. The amount of the composite oxide supported as the NOx storage material is preferably 0.54 to 54 g / L with respect to the catalyst carrier.

  The NOx storage material is supported on the catalyst support layer. This support means not only that the NOx storage material is disposed on the catalyst support layer but also that it is mixed with the catalyst support layer. That is, the exhaust gas purifying catalyst of the present invention has a porous metal oxide supporting a noble metal catalyst and a composite oxide represented by the above formula or a noble metal catalyst supported. It can be produced by precipitating the composite oxide represented by the above formula on the porous metal oxide.

  The exhaust gas purifying catalyst of the present invention may be a so-called pellet type catalyst, but is generally used as a monolith type catalyst in which a catalyst support layer is wash-coated on a support substrate. As the carrier base material, known base materials used for exhaust gas purification catalysts can be used. For example, heat-resistant ceramic materials such as cordierite, alumina, zirconia, silicon carbide, stainless steel, etc. It is preferable to use a honeycomb substrate made of the above metal, and it is particularly preferable to use a cordierite honeycomb having excellent heat resistance and a low coefficient of thermal expansion. This honeycomb substrate preferably has a large number of cells open at both ends. In this case, the cell density of the honeycomb substrate is not particularly limited, but it is preferable to use a so-called medium-density honeycomb of about 200 cells / square inch or a so-called high-density honeycomb substrate of 1000 cells / square inch or more, The cross-sectional shape of the cell is not particularly limited, and may be a circle, a rectangle, a hexagon, a circle, or the like.

  In a conventional NOx occlusion reduction type exhaust gas purification catalyst, an alkali or alkaline earth metal as a NOx occlusion material is supported as a carbonate or acetate. This alkali metal or alkaline earth metal reacts or dissolves with alumina or the like of the catalyst support layer or cordierite or the like as the support substrate at a high temperature of 600 ° C. or higher. It is considered that the NOx occlusion performance at high temperatures is lowered in the case of alkali metals or alkaline earth metals supported in the form of aluminum or acetate.

  On the other hand, in the present invention, a composite oxide of alkali metal-alkaline earth metal represented by the above formula is used as the NOx storage material, and this composite oxide is stable even at high temperatures. In addition, the alkaline earth metal can suppress a decrease in NOx occlusion performance at a high temperature without reacting or dissolving with the catalyst support layer even at a temperature of 600 ° C. or higher.

Example 1: Synthesis of K ₆ MgO ₄ / Pt / alumina Dinitrodiamine platinum chemical solution (9.09 g) was added to ion-exchanged water in which alumina powder (20 g) was dispersed, and mixed at room temperature for 1 hour. Thereafter, this mixture was dried at 120 ° C. for 2 hours, pulverized in a mortar, and calcined at 500 ° C. for 2 hours to obtain an alumina powder carrying platinum. 20 g of this powder was added to ion-exchanged water and stirred, while 14.72 g of potassium acetate and 3.56 g of magnesium acetate were added, and stirred at room temperature for 1 hour. Then, it was dried at 120 ° C. for 2 hours or more, pulverized in a mortar, and calcined at 750 ° C. for 2 hours to obtain a powder of K ₆ MgO ₄ (0.025 mol) / Pt (2 g) / alumina (100 g). This powder was hardened by pressure and pulverized on a sieve to obtain pellet particles having a diameter of 0.5 mm to 1 mm.

Comparative Example 1: Synthesis of K ₂ Ti ₄ O ₉ / Pt / alumina Dinitrodiamine platinum chemical solution (9.09 g) was added to ion-exchanged water in which alumina powder (20 g) was dispersed, and mixed at room temperature for 1 hour. Thereafter, this mixture was dried at 120 ° C. for 2 hours, pulverized in a mortar, and calcined at 500 ° C. for 2 hours to obtain an alumina powder carrying platinum. 20 g of this powder was added to ion-exchanged water and stirred. During that time, 2.94 g of potassium acetate and 4.79 g of fine titanium oxide powder (ST-01, manufactured by Ishihara Sangyo Co., Ltd.) were added and stirred at room temperature for 1 hour. Thereafter, it was dried at 120 ° C. for 2 hours or more, pulverized in a mortar, and calcined at 750 ° C. for 5 hours to obtain a powder of K ₂ Ti ₄ O ₉ (0.075 mol) / Pt (2 g) / alumina (100 g). . This powder was hardened by pressure and pulverized on a sieve to obtain pellet particles having a diameter of 0.5 mm to 1 mm.

Comparative Example 2: Synthesis of K / Pt / alumina Dinitrodiamine platinum drug solution (9.09 g) was added to ion-exchanged water in which alumina powder (20 g) was dispersed, and mixed at room temperature for 1 hour. Thereafter, this mixture was dried at 120 ° C. for 2 hours, pulverized in a mortar, and calcined at 500 ° C. for 2 hours to obtain an alumina powder carrying platinum. 20 g of this powder was added to ion-exchanged water and stirred, while 2.94 gg of potassium acetate was added and stirred at room temperature for 1 hour. Thereafter, it was dried at 120 ° C. for 2 hours or more, pulverized in a mortar, and calcined at 750 ° C. for 2 hours to obtain a powder of K (0.15 mol) / Pt (2 g) / alumina (100 g). This powder was hardened by pressure and pulverized on a sieve to obtain pellet particles having a diameter of 0.5 mm to 1 mm.

Evaluation of NOx purification performance 1.5 g of each pellet particle was charged into a fixed bed flow reactor (gas flow rate 6.6 (L / mim)), and the NOx purification performance was evaluated under the following conditions.
(i) Initial performance
After 10 minutes of rich treatment at 500 ° C., lean / rich = 60/6 sec was repeated 10 times at 400 ° C. When the waveform of the output NO concentration (ppm) was stabilized, the NO purification rate was evaluated. This NOx purification rate was calculated by the following equation.
NO purification rate = (Contained NO concentration (400ppm)-Out NO concentration (ppm)) x 100 / (Contained NO concentration (500ppm))

The gas composition is as follows.

(ii) After heat endurance
About the pellet particle | grains which baked in the air for 5 hours at 800 degreeC, NOx purification performance was evaluated similarly to the initial stage performance.

  The above results are shown in FIG. As is clear from the results shown in FIG. 1, the exhaust gas purifying catalyst of the present invention is suppressed from decreasing NOx storage activity after heat endurance. When the sample after durability of K / Pt / alumina of Comparative Example 3 was observed by XRD, it was confirmed that a composite oxide of potassium and alumina was formed.

  The exhaust gas purifying catalyst of the present invention can effectively remove NOx in the exhaust gas even after thermal endurance by being installed in the engine exhaust passage in the exhaust gas purifying apparatus for an internal combustion engine.

It is a graph which shows a NOx purification rate.

Claims (1)

An exhaust gas purification catalyst comprising a catalyst support layer, a noble metal catalyst supported on the catalyst support layer, and a NOx storage material, wherein the NOx storage material is represented by the following formula
K ₆ MgO ₄
A catalyst for purifying exhaust gas, characterized by being a composite oxide represented by the formula:

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